Abs are stabelizer muscles, not contracting muscles. Do planks. Obviously do sit ups to pratice but, planks will strengthen. Sit ups wont. Sit ups are more a measure of strength than a builder of strength.

Sit ups tend to be more hip flexor than abdominal, especially with the feet anchored. Abdominals are just as much movement muscles as stabilizers, so I'd do both: planks and continuing to practice unanchored by pressing your heels into the floor.

Try them and you will find out! They are an isometric exercise. Basically, you're resisting the force of gravity on your core by keeping it straight instead of letting it sag as it would if you totally relaxed your core. Isometrics don't look like anything is happening, but you can generate some crazy tension with them.

Agreed with planks. Download Mark's PB Fitness e-book and do the progression he suggests. Spidermans... side-planks... up-downs... these are all things that Mark has in his book and my personal trainer had me do (back when I had a trainer... sigh...). They're awesome for your abs!

Also, don't forget to do back exercises, too. Your "core" includes abs, obliques and lower back -- increase your strength in your abs and you can weaken your back. Do some back exercises to even things out.

If you have no problem with the anchored version then there may be a simple technical issue here. Try extending your legs a little further in front of you (and thus opening up your hip flexors) - this can sometimes help. A video of your technique would also help.

My situp skills improved drastically with pullups and leg raises. Laying on the floor and raising your legs (even if they are bent) up off the floor is a good place to start as well. The straighter your legs, the harder it gets. When you can do it with straight legs, then you can move on to hanging from a bar and lifting your legs. I've done a lot of situps over the years, but leg raises improved my numbers much more than simply trying to do more situps. It used to be that I couldn't do any situps without anchored feet, but now I could probably pass the Army test without. I use a lot less hip flexor than I used to as well.

Is there a requirement on how bent your legs have to be? The Army requires that your knees be at a 90 degree angle or tighter, but they require someone to hold your feet. The farther out your feet are, the easier it gets. I've never seen a situp test that prohibits anchoring the feet. What is this test for? Do they have an official description of how the situps are supposed to be done? What else is in this test? Can you wear heavy shoes just for this event? What, exactly, do you feel is your problem with this event? How many are you supposed to do? How many can you do? What is the time allotted?